The kits choose which exploits to load from their arsenal depending on the visitor's browser and installed plug-ins. If successful, the exploits install malware. It's known as a drive-by-download attack and is typically transparent to users.

Exploit kits usually target known vulnerabilities, which is why it's important to keep browser plug-ins like Flash Player up to date. But that doesn't help if attackers have an exploit for a zero-day vulnerability -- one that's not been patched yet by the software vendor.

Zero-day exploits are valuable to hackers, which is why they're more commonly used in targeted attacks where the stakes are higher and the goal is usually cyberespionage. It's unusual to see them in mass attacks like those performed with Angler and other exploit kits.

The new zero-day exploit used in Angler worked successfully on the latest Flash Player version on Windows 7 with Internet Explorer 8, Windows 8 with Internet Explorer 10, and Windows XP with IE 6 to 9, Kafeine said in a blog post. Disabling Flash Player for some days might be a good idea, he said.

Adobe is aware of the report and investigating the claims, a spokeswoman said via email.

However, Kafeine claims he shared the exploit with the company, and that Adobe has confirmed it.

"The zero-day vulnerability in Flash Player, as discovered by Kafeine, could provide a big security risk for Internet users, effectively opening an unguarded window onto PCs worldwide," said Pedro Bustamante, director of special projects at security firm Malwarebytes, via email.

"The fact that it has seemingly been integrated into the Angler Exploit Kit shows that criminals are keen to use it to target people and businesses en-masse," he said. "Using a delivery mechanism such as Angler increases the chance of successful infections, allowing for accurate attacks through infected adverts on high traffic websites."

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